This invention relates generally to pressure vessels and in particular to pressure vessels for withstanding cyclic pressure and temperature extremes such as those found in nuclear reactor pressure vessels.
Pressure vessels used for containing a nuclear reactor must be designed for operating conditions involving a combination of high temperatures and high pressures along with strong radiation in the form of neutrons and gamma rays.
The pressure vessel must also be designed for emergency conditions such as Loss of Coolant Accidents (LOCA) and Pressurized Thermal Shock (PTS).
The LOCA conditions may cause the vessel pressure to drop suddenly while the temperature may remain the same or even increase
The PTS condition, which may be caused by the injection of cold "coolant" into the pressure vessel from the emergency core cooling system (ECCS), may cause steep temperature gradients, such as, a sudden drop in temperature of the inside surface of the pressure vessel and a portion of the vessel wall, while the pressure may remain constant or even increase.
When pressure vessels are subjected to these pressure and temperature extremes, high tensile stresses are produced inside the vessel wall, particularly in the region of wall penetrations, such as, coolant inlet and outlet ports of the pressure vessel. During pressurized thermal shock (PTS) (as might be caused by a loss of coolant accident (LOCA)) where cold coolant is introduced to replace the lost coolant, steep thermal gradients in the vessel wall may cause large tensile stresses to occur in the crotch region of the vessel-outlet and vessel-inlet port intersections which may result in cracking of the vessel wall.
In addition, crack initiation may be aided due to embrittlement of the vessel wall by the high level of neutron and gamma radiation being absorbed.
To alleviate this condition, some multiple layer pressure vessels of the prior art utilized concentrically disposed multiple shell pressure vessels shrunk fit onto each other.
Other pressure vessels utilized multiple layers of sheet metal spirally wrapped around the outside of an inner pressure vessel.
In some of the pressure vessels using spaced apart shells, the space between the shell was filled with a neutron absorbing material.
One prior art device utilized spaced apart pressure vessel shells filled with coolant or a low melting point material to effect a uniform pressure distribution. The filler material was not maintained under pressure.